1. Field of the Invention
The present invention relates to high resolution optical scanners, and more particularly, to an optical scanner which corrects for alignment errors of a scanning polygon.
2. State of the Prior Art
High resolution optical scanners can be defined as scanners which produce from 10,000-50,000 resolutions, or spots, per scan. Such scanners are used in the graphics arts for scanning an image to be recorded and for printing on a receiving medium. Certain high resolution scanners move an optical head such that the optical system itself is essentially an on-axis system. These scanners are relatively slow compared with scanners which deflect a laser beam across the medium. However, in the deflecting-type scanners complex optical designs are necessary to achieve a large number of resolvable spots on the receiving medium.
In U.S. Pat. No. 4,247,160, there is disclosed a laser beam scanner having a positive cylinder mirror located between the polygon deflector and a receiving medium. The positive cylinder mirror, which has power in the sagittal plane but has no power in the tangential plane, minimizes spot position errors due to alignment errors between adjacent facets of the polygon deflector. One problem with the patented scanner is that corrective optical elements must be used in combination with the cylinder mirror as, for example, a post-deflector lens with appropriate power in the tangential plane. These optical elements, in addition to adding to the complexity of the device, also introduce problems inherent in lenses, such as chromatic aberrations, which tend to limit the effectiveness of the scanner for high resolution scanning.